1. Technical Field
This disclosure relates generally to analog-to-digital signal conversion and in particular, but not exclusively, to image sensor readout circuitry.
2. Background Art
Image sensors have become ubiquitous. They are widely used in digital still cameras, cellular phones, security cameras, as well as, medical, automobile, and other applications. The technology used to manufacture image sensors, and in particular, complementary metal oxide semiconductor (CMOS) image sensors, has continued to advance at a great pace. For example, the demands of higher resolution, high quality images and lower power consumption have encouraged the further miniaturization and integration of these CMOS image sensors. However, fixed pattern noise—or “FPN”—is a known issue for CMOS (and other) image sensors. FPN is a spatial variation in pixel outputs under uniform illumination due to device and interconnect mismatches within an image sensor. FPN may present itself in a resultant image as some pattern of brighter or dimmer pixels occurring with images taken under the same temperature and exposure. Column FPN (CFPN) is a common name for a specific type of FPN that is due to the variation in the amplifier or analog-to-digital converter (ADC) circuitry that is coupled along a pixel array “column” (which, in this context, refers to a direction along which an output bitline is shared by multiple pixels). CFPN present itself in a resultant image as some pattern of brighter or dimmer columns occurring with images taken under the same temperature and exposure.
Conventional methods of reducing FPN in image sensors include correlated double sampling (CDS), in which an analog reference signal (or black signal level) is amplified and then sampled before resetting a pixel cell. During subsequent image acquisition, the reset pixel cell is exposed to light and charged to produce an analog image signal. The analog image signal is amplified, then sampled and compared with the sampled reference signal (i.e., black signal is subtracted from the image signal) to arrive at a final value (i.e., the resultant image signal).
Amplification of such analog reference and image signals is performed with what is commonly referred to as column gain amplifier circuitry, which in conventional image sensors then provides an amplified analog output to analog-to-digital circuitry for further processing. CFPN caused by the variations in an ADC for a pixel array column is suppressed by the column gain amplifier by a factor of the column gain. As the demand for small form factor and/or low cost image sensor devices continues to grow there is a corresponding need for efficient solutions capable of mitigating CFPN in a variety of image sensing conditions without the cost of a column gain amplifier. Incremental improvements to CFPN mitigation are thus expected to become increasingly valuable for successive generations of image sensing technology.